1. Field of the Invention
The invention concerns a method for correction of MR signals that have been acquired with a time-of-flight (TOF) MR angiography technique, as well as an MR system to implement the correction.
2. Description of the Prior Art
In time-of-flight MR angiography, images of the vessel structure of an examination subject are generated with the use of gradient echo sequences. This angiography method is based on the fact that spins that remain stationary in a slice are saturated by many RF pulses, and therefore deliver little signal, while spins that flow perpendicular to the excited slice are replaced with new spins that can deliver markedly more signal. A high contrast between vessels and the remaining stationary tissue that supplies the background signal can be generated based on this phenomenon.
In the case of the excitation of a three-dimensional target volume that contains more than one slice, multiple RF excitation pulses that depend on the flow speed and the slice thickness of the individual slices in the target volume act on the blood flowing in. In order to prevent the signal intensity in the vessels from decreasing due to a saturation of the spins flowing into the target volume, the flip angles are varied across the target volume in order to obtain a homogeneous vessel signal across said target volume. This variation of the flip angles in the excitation of the spins, however, leads to a gradient in the signal intensity curve in the pixels with background signal. In the present case, “pixels with background signal” means those pixels that do not show a vessel in the MR angiography image and do not show the noise outside of the examined tissue, but rather the non-flowing tissue surrounding the vessel. Usually, acquisitions with multiple overlapping target volumes are necessary to show a vessel structure. If multiple overlapping target volumes are now acquired, respective signal intensity gradients result in the pixels with background signal in the different target volumes. In the assembled MR angiography image of the individual target volumes, this now leads to edge artifacts between the individual target volumes, known as the venetian blind artifact.
In “Sliding Interleaved ky (SLINKY) Acquisition: A Novel 3D MRA Technique with Suppressed Slab Boundary Artifact” by Kecheng, Liu et al. in JMRI 1998; 8, Pages 905-911, a new acquisition strategy is proposed to reduce this artifact, but this technique has not yet become established in practice.